A tooth may develop a carious lesion. A carious lesion infects tooth tissue. A carious lesion infects tooth tissue in a root canal of the tooth. If tooth tissue in the root canal is infected, the infected tissue should be removed from the tooth to stop further spreading of the carious lesion and/or infection.
A dental procedure for removing infected tooth tissue from a root canal typically requires specialized tools. Such tools include an endodontic file. Endodontic files are typically used to remove infected tooth tissue from the root canal. Endodontic files are also used to remove infected tooth tissue from other parts of the tooth such as tissue adjacent to the root canal.
A shape of a root canal differs from patient to patient. Typically, the canal is narrow and tortuous. Root canal curvatures are typically divided in three categories: straight, moderate and severe. Endodontic files are flexible to navigate a curvature of the root canal.
Tables 1 and 2 (below) summarize averages of root canal configuration and lengths.
TABLE 1Illustrative root canal configurationsToothAverage LengthNo. of rootsNo. of canalsMaxillaryanteriorsCentral incisor22.5 mm11Lateral incisor22.0 mm11Canine26.5 mm11MaxillarypremolarFirst premolar20.6 mm2-31(6%)2(95%)3(1%)Second premolar21.5 mm1-31(75%)2(24%)3(1%)MaxillarymolarsFirst molar20.8 mm34(93%)3(7%)Second molar20.0 mm34(37%)3(63%)Third molar17.0 mm1-3MandibularanteriorsCentral incisor20.7 mm11(58%)2(42%)Lateral incisor20.7 mm1-21(58%)2(42%)Canine25.611(94%)2(6%)MandibularmolarsFirst molar21.0 mm2-33(67%)4(33%)Second molar19.8 mm22(13%)3(79%)4(8%)Third molar18.5 mm1-2
TABLE 2Illustrative human teeth meaurementsLengthLength ofLength ofToothoverallcrownrootMaxillaryanteriorsCentral22.51012incisorLateral228.813incisorCanine26.59.517.3MaxillarypremolarFirst premolar20.68.212.4Second21.57.514premolarMaxillarymolarsFirst molar20.87.713.2Second molar207.213Third molar17.16.311.4MandibularanteriorsCentral20.78.811.8incisorLateral21.19.612.7incisorCanine25.610.315.3MandibularpremolarFirst premolar21.67.814Second22.37.914.4premolarMandibularmolarsFirst molar217.713.2Second molar19.86.912.9Third molar15.86.711.8Average of root canal length: 13.5
Endodontic files are described using the following nomenclature and associated equations:A=TipB=TaperL1=length at 1 mm from the end of the fileL10=length at 10 mm from the end of the fileD1=file's diameter @L1=A(Tip)D10=file's diameter @L10=B*10+(D1)β=file's taper=TAN−1(((D10/2)−(D1/2))/9)
Endodontic files used to remove infected tissue from a root canal are small enough to remove infected tissue from the canal without damaging uninfected tooth tissue. Typical file lengths include 23 and 27 mm. Such lengths address short and long canals. Endodontic files are also tapered to “fit” into a root canal and reach an apical foramen of the canal. Some files may not be tapered. Table 3 below shows common file sizes.
TABLE 3Common File SizesCommon File Sizes measured in hundredths of a millimeter at thetip of the instrument (Do) (e.g., size 60, Do = 60/100 = 0.6 mm)68101112131415202530354045505560708090100110120130140
FIG. 1 shows illustrative taper designs and associated measurements.
Endodontic files may be rotated to remove infected tissue. The files are rotated by hand and/or machine. It is preferable to shape a canal in manner that, as close as possible, maintains the dimensions and/or proportionality of the original canal. Such a shaped canal reduces loss of strength in the surrounding tooth structure and allows the canal to later be filled during an obturation phase with as few voids as possible
A desired apical size and taper includes shaping the canal into a conical or at least partially conical shape. The conical shape extends from an access cavity (at a coronal area of the tooth) to the apical foramen of the tooth. A conically shaped root canal facilitates removal of dentinal debris (e.g., necrotic tissue), and cleaning of the canal during an irrigation process of a dental procedure. The conical shape also facilitates proper flow of gutta-percha or other suitable sealants during a subsequent obturation phase of the root canal dental procedure.
However, because of their small size, endodontic files are susceptible to breaking during operation in the canal. For example, in operation, when navigating curved sections of the canal, a file is exposed to torsion and bending stresses. Such stresses are associated with a file breakage or other failures. Breakage of the file in the canal during the dental procedure is associated with undesirable complications. For example, it is difficult to extract a piece of the broken file from the canal.
Endodontic files are constructed from various materials, including stainless steel and Nickel Titanium (“NiTi”). NiTi is an alloy that typically exists in two crystal structures, austenite and martensite. NiTi also exists in a third crystal structure termed a rhombohedral phase or r-phase. A change between different NiTi crystal structures may be stress-induced or thermally induced. An ability of NiTi to transition from one crystal structure to the other provides NiTi files with superelastic and shape memory properties.
NiTi's superelasticity and high degree of flexibility make it an attractive choice for endodontic file fabrication. A highly elastic file remains centered in the root canal and has a lower propensity of canal straightening or other preparation errors. NiTi's shape memory provides an endodontic file with an ability to resist deformation in response to stresses applied to the file when operating in the canal.
Additionally, rotational movement in curved canals bends a file once per revolution, exposing the file to cyclic fatigue. More flexible files are less susceptible to cyclic fatigue and therefore less likely to break during a dental procedure. However, even superelastic files are not immune from cyclic fatigue.
To reduce a risk of file breakage, during the dental procedure, a set of NiTi files is typically used from relatively small to progressively larger sizes. To reduce a risk of file breakage, a file is advanced until a threshold resistance is encountered. The file is then withdrawn and a larger size file is introduced and advanced further apically. The procedure progresses sequentially until the canal is fully prepared.
After the canal has been shaped to a desired size and taper, gutta-percha or another suitable sealant is introduced into the prepared canal during a subsequent obturation phase of the dental procedure.
It would be desirable to provide an endodontic file that is even more resistant to torsional stress and cyclic fatigue when operating in the canal. It would be desirable to provide an endodontic file that balances superelasticity and shape memory properties for a specific function and/or size associated with the file.